<h2>DESCRIPTION</h2>

<em>r.mfilter</em> filters the raster <em>input</em> to produce the
raster <em>output</em> according to the matrix <em>filter</em> designed
by the user (see <em>FILTERS</em> below).
The filter is applied <em>repeat</em> times (default <em>value</em> is 1).
The <em>output</em> raster map layer can be given a <em>TITLE</em> if desired.
(This TITLE should be put in quotes if it contains more than one word.)

With <b>-z</b> flag the filter is applied only to null values in
the input raster map layer.  The non-null category values are not changed.
Note that if there is more than one filter step, this rule is applied to the
intermediate raster map layer -- only null category values which result from
the first filter will be changed.  In most cases this will NOT be the
desired result. Hence -z should be used only with single step filters.
<p>
The <b>filter</b> parameter defines the name of an existing, user-created
UNIX ASCII file whose contents is a matrix defining the way in which the
<em>input</em> file will be filtered. The format of this file is described
below, under FILTERS.
<p>
The <b>repeat</b> parameter defines the number of times the <em>filter</em>
is to be applied to the <em>input</em> data.

<h2>FILTERS</h2>

The <em>filter</em> file is a normal UNIX ASCII file designed by the user.
It has the following format:
<div class="code"><pre>
     TITLE      TITLE
     MATRIX     n
                  .
     n lines of n values
                  .
     DIVISOR    d
     TYPE        S/P
</pre></div>

<dl>
<dt>TITLE 

<dd>A one-line TITLE for the filter.
If a TITLE was not specified on the command line, it can be specified here.
This TITLE would be used to construct a TITLE for the resulting raster map
layer.  It should be a one-line description of the filter.

<dt>MATRIX 

<dd>The matrix (n x n) follows on the next n lines.  <em>n</em> must be
an odd integer greater than or equal to 3.
The matrix itself consists of n rows of n values.
The values must be separated from each other by at least 1 blank.

<dt>DIVISOR 

<dd>The filter divisor is <em>d</em>.  If not specified, the default is 1.
If the divisor is zero (0), then the divisor is dependent on the
category values in the neighborhood
(see HOW THE FILTER WORKS below).

<dt>TYPE 

<dd>The filter type.  <em>S</em> means sequential, while <em>P</em> mean parallel.
If not specified, the default is S.


<p>
Sequential filtering happens in place.  As the filter is applied to the
raster map layer, the category values that were changed in neighboring
cells affect the resulting category value of the current
cell being filtered.


<p>Parallel filtering happens in such a way that the original raster
map layer category values are used to produce the new category value.


<p>More than one filter may be specified in the filter file.
The additional filter(s) are described just like the first.
For example, the following describes two filters:


</dl>


<h2>EXAMPLE FILTER FILE</h2>

<div class="code"><pre>
      TITLE     3x3 average, non-null data only, followed by 5x5 average
     MATRIX    3
     1 1 1
     1 1 1
     1 1 1
     DIVISOR   0
     TYPE      P

     MATRIX    5
     1 1 1 1 1
     1 1 1 1 1
     1 1 1 1 1
     1 1 1 1 1
     1 1 1 1 1
     DIVISOR   25
     TYPE      P
</pre></div>

<h2>HOW THE FILTER WORKS</h2>

The filter process produces a new category value for each cell
in the input raster map layer by multiplying the category values of the
cells in the n x n neighborhood around the center cell
by the corresponding matrix value and adding them together.
If a divisor is specified, the sum is divided by this divisor.
(If a zero divisor was specified, then
the divisor is computed for each cell as the sum of the MATRIX
values where the corresponding input cell is non-null.)


<p>
If more than one filter step is specified, either because the
repeat value was greater than one or because the filter file
contained more than one matrix, these steps are performed
sequentially. This means that first one filter is applied to
the entire input raster map layer to produce an intermediate result;
then the next filter is applied to the intermediate result to
produce another intermediate result;  and so on, until the
final filter is applied.  Then the output cell is written.

<h2>NOTES</h2>

If the resolution of the geographic region does not agree with the
resolution of the raster map layer, unintended resampling of the original
data may occur.  The user should be sure that the geographic region
is set properly.

<h2>SEE ALSO</h2>

<em>
<a href="g.region.html">g.region</a>,
<a href="r.clump.html">r.clump</a>,
<a href="r.neighbors.html">r.neighbors</a>,
<a href="r.resamp.filter.html">r.resamp.filter</a>
</em>

<h2>AUTHOR</h2>

Glynn Clements.
Based upon r.mfilter, by Michael Shapiro,
U.S.Army Construction Engineering Research Laboratory

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